A new chapter penned by unassuming boy (3-5)

I believe the answer is:

man-child

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'boy' is the definition.
(I know that boy can be written as man-child)

'a new chapter penned by unassuming' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'chapter' becomes 'ch'.
'penned by' means one lot of letters goes inside another (inserted letters are penned in).
'unassuming' becomes 'mild' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'n'+'ch'='anch'
'anch' put into 'mild' is 'MAN-CHILD'.

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I've seen this clue in The Independent.

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